Constant velocity joints typically used in the drive system of vehicles for transferring a rotational torque, are classified typically into the fixed type joint allowing only angular displacement between the axles and the plunging type joint allowing both angular and axial displacement between the axles. The fixed type constant velocity joint is required to operate with a relatively high operating angle of, for example, 45° or more, as compared to the plunging type joint.
FIG. 1 illustrates a conventional fixed type constant velocity joint known as a Rzeppa type constant velocity joint (referred hereinafter as a “Rzeppa joint”). This joint includes an outer race 11 having a spherical inner space formed with a plurality of (i.e., six) outer ball grooves 111, and an inner race 12 with a plurality of (i.e., six) corresponding inner ball grooves 121. A plurality of (i.e., six) torque transmitting balls 13 are received and guided in a respective track defined by a outer ball groove 111 and its corresponding inner ball groove 121, and a cage 14 which has a plurality of (i.e., six) cage windows 141 to hold the balls 13 in a same plane.
In the Rzeppa joint, the ball guide grooves 111 and 121 of the outer and inner races 11 and 12 each have a curved contact surface curved with a radius, and the centers C′ and C″ of the outer and inner guide grooves 111 and 121 are respectively offset with respective to the spherical joint center C0 of the outer and inner race 13 and 12 by a same distance “f” in opposite directions. The center offset f of outer ball grooves 111 and inner ball grooves 121 is applied to maintain a constant velocity characteristic and accommodate a smooth movement of balls 13 when the outer and inner joints are articulated with respect to each other. Funnel angles θf are defined as the angles between the tangential lines at the ball contact points in the outer ball grooves 111 and inner ball grooves 121 when the joint is in a specific joint operating angle. The funnel angle θf is decided typically by the pitch circle radius (PCR) of the ball and center offset f of outer ball groove 111 and inner ball groove 121. As the ball 13 is pressed in the grooves 111 and 121 with the funnel angle θf present, a resultant axial force F is applied onto the balls, and consequently, against the cage 14 as the contact force. Thus, a selection of center offset f and funnel angle θf becomes an important factor for the determination of the strength and durability of the joints.
FIG. 1(b) illustrates the funnel angle θf of the Rzeppa joint at the operating angle θ. In this structure, the funnel angles θf of upper ball 13u and lower ball 13w are the same, and funnel angles θf are constant within the joint operating angle θ. FIG. 1(c) illustrates the directions of ball contact forces F for each groove in the typical Rzeppa Joint. The forces F1, F2, F3 and F4 have the same direction and a total ball contact force is calculated as a sum of these forces.
FIG. 2 illustrates a typical shape of cage 14 which has cage windows 141, aligned in a common plane to hold the balls 13 in the same plane. In general, the strength or durability of cage 14 in the fixed type joint is determined by the stress on a web area 142 (i.e., the structural area between the two adjacent cage windows CW), in which the web stress is defined as the ball contact force F (present due to the funnel angle θf) per web area 142. In addition, cage web area 142 is determined by the width of cage windows CW, which are designed to have a size to cover the entire ball movement range during assembly (with minimum assembly angle θa present) and also in operation (with a maximum operating angle θ of joint present). Thus, an optimization in the funnel angle θf and ball movement through selection of an appropriate center offset f is required to improve the strength and durability of cage 14.
FIG. 3(a) illustrates the state when the joints are articulated for assembly. In a typical fixed type joint, the maximum ball movement occurs when the ball 13 assembles to the joint, because the operating angle θ is designed to have an angle less than the assembly angle θa. The minimum assembly angle θa is defined as the angle when the specific ball diameter DB is less than the clearance δ1 between the edge of cage window 141 and the outer edge of outer race groove 111. Therefore, to reduce the minimum assembly angle θa, a distance Dz from the joint center line L0 to the edge of outer race groove 111 needs to be enlarged, and thus, causing to disadvantageously increase the size of the outer race 11 and the joint.
FIG. 3(b) illustrates movements (Mw) of the balls when the joint is in the minimum assembly angle θa. With a joint angle present, the balls 13 assume different angular positions within the cage 14 depending on the angular position of the particular groove relative to the neutral (reference) plane PA of articulation. In this figure, if the minimum assembly angle θa is increased, the maximum ball movement MW (which is measured typically from the center 14d of the cage window) also increases, and consequently, the cage window 141 must also have an increased width CW to obtain the minimum clearance δ2 between ball 13 and a corresponding side of cage web 142, which leads to the reduction of web area 142, and this causes the deterioration in strength of cage 14.
As described above, because the conventional fixed type joint has the above described limitations to the cage design, it is difficult to enlarge the thickness of the cage web area 142 to improve the mechanical strength of the cage which is subject to repeated stresses during operation, and thus, limiting to the ability to design the joint in compact size. Moreover, because the conventional design requires a minimum window size dictated by the necessary minimum assembly angle θa, the volume of the web cage (and thus, the joint) becomes proportionally increased if the conventional joint structure is applied to the high-ball joint with more balls than the traditional six-ball type joint. Thus, because the size of the joint is typically limited in order not to increase the volume and weight of an automobile, the conventional joint design is disadvantageous to apply to the fixed type joint having more than six balls, such as eight or ten balls.